The principle analysis of JAVA HashMap

Let's take a look at the structure of the hashmap as a whole. As shown in the following figure (the picture is not good), the box represents the hash bucket, the ellipse represents the elements in the barrel, here is the key-value to the composition of the map.entry image.

If there are multiple elements of the hash function to locate the same bucket, we call it a hash conflict, the element in the bucket is composed of one-way linked list. Let's take a look at the HashMap JDK source code (due to the length of the relationship, delete some of the codes and comments, can be viewed JDK1.6 source):

public class hashmap<k,v>


extends Abstractmap<k,v>


implements Map<k,v&gt, Cloneable, Serializable


{


static final int default_initial_capacity = 16;


static final int maximum_capacity = 1 << 30;


static final float default_load_factor = 0.75f;


transient entry[] table;


transient int size;


int threshold;


final float Loadfactor;


transient volatile int modcount;


public HashMap (int initialcapacity, float loadfactor) {


if (initialcapacity < 0)


throw new IllegalArgumentException ("Illegal initial capacity:" +


initialcapacity);


if (initialcapacity > Maximum_capacity)


initialcapacity = maximum_capacity;


if (loadfactor <= 0 | | Float.isnan (loadfactor))


throw new IllegalArgumentException ("Illegal load factor:" +


loadfactor);


//Find a power of 2 >= initialcapacity


int capacity = 1;


while (capacity < initialcapacity)


capacity <<= 1;


this.loadfactor = Loadfactor;


threshold = (int) (capacity * loadfactor);


table = new Entry[capacity];


init ();


　　　　 }





public V get (Object key) {


if (key = = null)


return Getfornullkey ();


int hash = hash (Key.hashcode ());


for (entry<k,v> e = table[indexfor (hash, table.length)];


e!= null;


e = e.next) {


Object K;


if (E.hash = = Hash && ((k = e.key) = = Key | | key.equals (k))


return e.value;


　　　　　　　　 }


return null;


　　　　 }


private V Getfornullkey () {


for (entry<k,v> e = table[0]; e!= null; e = e.next) {


if (E.key = null)


return e.value;


　　　　　　　　 }


return null;


　　　　 }


Public V-Put (K key, V value) {


if (key = = null)


return Putfornullkey (value);


int hash = hash (Key.hashcode ());


int i = indexfor (hash, table.length);


for (entry<k,v> e = table[i]; e!= null; e = e.next) {


Object K;


if (E.hash = = Hash && ((k = e.key) = = Key | | key.equals (k))) {


V oldValue = e.value;


E.value = value;


e.recordaccess (this);


return oldValue;


　　　　　　　　　　　　 }


　　　　　　　　 }


modcount++;


addentry (hash, key, value, I);


return null;


　　　　 }


Private v Putfornullkey (v value) {


for (entry<k,v> e = table[0]; e!= null; e = e.next) {


if (E.key = = null) {


V oldValue = e.value;


E.value = value;


e.recordaccess (this);


return oldValue;


　　　　　　　　　　　　 }


　　　　　　　　 }


modcount++;


addentry (0, NULL, value, 0);


return null;


　　　　 }


}